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Physics of Dust and Sand Storms
Alee K. Obeid
BSc. Mechanical Engineering (Aeronautics) High Level Post Graduate Aerospace Engineering Student
1. Introduction The wind-driven emission, transport, and deposition of sand and dust by wind are termed
Aeolian processes, after the Greek god Aeolus, the keeper of the winds. Aeolian processes occur
wherever there is a supply of granular material and atmospheric winds of sufficient strength to
move them. On Earth, this occurs mainly in deserts, on beaches, and in other sparsely vegetated
areas, such as dry lake beds. The blowing of sand and dust in these regions helps shape the
surface through the formation of sand dunes and ripples, the erosion of rocks, and the creation
and transport of soil particles. Moreover, airborne dust particles can be transported thousands of
kilometers from their source region, thereby affecting weather and climate, ecosystem
productivity, the hydrological cycle, and various other components of the Earth system.[Ref.(1)].
But Aeolian processes are not confined to Earth, and also occur on Mars, Venus, and the
Saturnian moon Titan (Greeley and Iversen 1985). On Mars, dust storms occasionally obscure the
sun over entire regions of the planet for days at a time, while their smaller cousins, dust devils,
punctuate the mostly clear daytime skies elsewhere (Balme and Greeley 2006). The surface of
Mars also hosts extensive fields of barchans, transverse, longitudinal, and star-like dunes, as well
as other exotic dune shapes that have not been documented on Earth (Bourke et al. 2010). On
Venus, transverse dunes have been identified by the Magellan orbiter (Weitz et al. 1994), while
the Cassini orbiter has documented extensive longitudinal sand dunes on Titan (Lorenz et al.
2006). [Ref. (1)].
The terms dust and sand usually refer to solid inorganic particles that are derived from
the weathering of rocks. In the geological sciences, sand is defined as mineral (i.e., rock-derived)
particles with diameters between 62.5 and 2,000 m, whereas dust is defined as particles with
diameters smaller than 62.5 m (note that the boundary of 62.5 m differs somewhat between
particle size classification schemes, see Shao 2008, p. 119). In the atmospheric sciences, dust is
usually defined as the material that can be readily suspended by wind (e.g., Shao 2008), whereas
sand is rarely suspended and can thus form sand dunes and ripples, which are collectively termed
bed forms. [Ref. (1)].
Dust storms are among the most severe environmental problems in certain regions of the
World. In where they occur most of the dust in the atmosphere is from Aeolian origin. Estimates
of the total Aeolian dust from deserts in the atmosphere are about ton/yr (Ning Ai and Karen R. Polenske). Several authors (JungeC 1979): Ganor E, Mamane Y 1982: Morales C 1979)
have estimated that the Sahara desert alone contributes ton/yr or between 4066% of the total dust. Dust storms may be traced as far as 4000 km from their origin. [Ref. (2)].
Dust storms may cause a variety of problems. One of the major problems is a
considerable reduction of visibility that limits various activities, increases traffic accidents, and
may increase the occurrence of vertigo in aircraft pilots (Morales C., 1979; Hagen L.J, Woodruff
N.P. 1973; Middleton N.J, Chaudhary QZ 1988; Dayan U, Heffter J, Miller J, Gutman G 1991;
Yong-Seung C, 1996). Other environmental impacts, reported in the literature (Hagen L.J,
Woodruff N.P. 1973; Mitchell J.M. 1971; Fryrear D.W. 1981; Victor R. Squires. 2007; Jauregui
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E. 1989; Liu C.M. Ou S.S. 1990; Yong-Seung C, Ma-Beong Y. 1996 and Ning Ai and Karen R.
Polenske) include reduced soil fertility and damage to crops, a reduction of solar radiation and in
consequence the efficiency of solar devices, damage to telecommunications and mechanical
systems, dirt, air pollution, increase of respiratory diseases and so on. Three main categories of
reduced visibility are often used to describe the severity of dust storms.
Blowing Dust the horizontal visibility (due to Aeolian dust), is less than 11 km. Dust Storm the horizontal visibility is less than 1000 m. Severe Dust Storm the horizontal visibility is less than 200 m. [Ref. (2)].
In some seasons in certain regions of the Middle East and North Africa and for about
30% of the time on average, the dust conditions in the lower troposphere fall into one of these
three categories. Thus, in these regions, dust storms are a very frequent phenomenon and a better
knowledge of their spatial and temporal distribution is of prime importance. A positive
correlation exists between the quantity of dust in the air, and the wind velocity. Whereas, a
negative correlation exists between dust amount and the particles size. Precipitation and/or vegetation coverage may reduce considerably the amount of dust in the air for a given wind
velocity and/or particles size (Bagnold R.A. 1941; Gillette D.A. 1979; Mitchell J.M. 1971). Thus, a study of the atmospheric circulation and its impact on the precipitation regime in a given
region is crucial to understand the dust distribution in that region. [Ref. (2)].
A previous attempt to delimit the regions in the Middle-East according to the seasons of
main activity was done by Middleton (Middleton N.J. 1988). He analyzed the dust distribution
over Syria, Lebanon, Jordan, Israel, Saudi Arabia, Yemen, Iraq and Iran and his analyses were
based on short periods of data recording and on other data collected over varying lengths of time
from the 1950s and 1960s. The present study extends Middletons analysis in three ways: (a) the study area was extended to include also data from Turkey, Cyprus, Egypt and Sudan, (b) the
analysis period was extended to 21 years (19731993) and (c) the clustering of the different stations into coherent regions was done in an objective way using cluster analysis. [Ref. (2)].
1.1 Manners of Wind-Blown particle transport
The transport of particles by wind can occur in several manners, which depend
predominantly on particle size and wind speed (Figure 1.1). As wind speed increases, sand
particles of ~100 m diameter are the first to be moved by fluid drag. After lifting, these particles
hop along the surface in a process known as saltation (Bagnold 1941, Shao 2008), from the Latin
salto, which means to leap or spring. The impact of these saltators on the soil surface can
mobilize particles of a wide range of sizes. Indeed, dust particles are not normally directly lifted
by wind because their interparticle cohesive forces are large compared to aerodynamic forces.
Instead, these small particles are predominantly ejected from the soil by the impacts of saltating
particles (Gillette et al. 1974, Shao et al. 1993a). Following ejection, dust particles are susceptible
to turbulent fluctuations and thus usually enter short-term (~ 20 - 70 m diameter) or long-term
(< ~20 m diameter) suspension (Figure 1.1). Long-term suspended dust can remain in the
atmosphere up to several weeks and can thus be transported thousands of kilometers from source
regions (Gillette and Walker 1977, Zender et al. 2003a, Miller et al. 2006). These dust aerosols
affect the Earth and Mars systems through a wide variety of interactions. [Ref.(1)].
The impacts of saltating particles can also mobilize larger particles. However, the
acceleration of particles with diameters in excess of ~500 m is strongly limited by their large
inertia, and these particles generally do not saltate (Shao, 2008). Instead, they usually settle back
to the soil after a short hop of generally less than a centimeter, in a manner of transport known as
reptation (Ungar and Haff 1987). Alternatively, larger particles can roll or slide along the surface,
driven by impacts of saltating particles and wind drag forces in a mode of transport known as
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creep (Bagnold 1937). Creep and reptation can account for a substantial fraction of the total wind-
blown sand flux (Bagnold 1937, Namikas 2003). [Ref.(1)].
The transport of soil particles by wind can thus be crudely separated into several physical
regimes: long-term suspension (< ~20 m diameter), short-term suspension (~20 70 m), saltation (~70 500 m), and reptation and creep (> ~500 m) (Figure 1.1). Note that these four transport modes are not discrete: each mode morphs continuously into the next with changing
wind speed, particle size, and soil size distribution. The divisions based on particle size between
these regimes are thus merely approximate. [Ref.(1)].
Figure 1.1. Schematic of the different modes of Aeolian transport. [Ref.(1)].
A recent study finds that the initial saltation of sand particles induces a static electric field
by friction. Saltating sand acquires a negative charge relative to the ground which in turn loosens
more sand particles which then begin saltating. This process has been found to double the number
of particles predicted by previous theories. (Electric Sand Findings 2008). [Ref. (2)].
1.2 Contribution of study of wind-blown sand and dust to the Earth and planetary sciences
Wind-blown sand has shaped a substantial portion of the Earths surface by creating sand dunes and ripples in both coastal and arid regions (Bagnold 1941, Pye and Tsoar 1990), and by